1. Field of the Invention
The present invention relates to a hot-rolled stainless steel strip (a generic term including steel sheet), and a method for producing the same. More particularly, the present invention relates to a hot-rolled stainless steel strip which can be worked by bending, drawing, etc., without first descaling the strip by pickling after hot-rolling, and which has excellent descalability if pickling is to be performed, as well as excellent surface properties after descaling, and a production method for such steel strip.
2. Description of the Related Art
A hot-rolled stainless steel strip is generally produced by hot-rolling a steel slab formed by continuous casting after heating at about 1100 to 1300.degree. C. The hot-rolled stainless steel strip is then subjected to continuous or batch annealing or passed through sulfuric acid bath and mixed nitric acid/hydrofluoric acid bath for pickling without annealing, and then cold-rolled to form a cold-rolled stainless steel strip. The cold-rolled stainless steel strip is further annealed and pickled, and then used for various applications.
On the other hand, in some cases, the hot-rolled stainless steel strip is annealed, pickled and then used for various applications without cold-rolling.
Since stainless steel contains much Cr, a Fe--Cr oxide layer mainly comprising (Fe,Cr).sub.2 O.sub.3 and (Fe,Cr).sub.3 O.sub.4 is formed on the surface of the steel strip during hot rolling, and an intermediate oxide layer comprising SiO.sub.2 is formed at the interface of the Fe--Cr oxide layer and the alloy substitute, due to Si present in the steel. Cold rolling after annealing of the hot-rolled steel strip having such oxide layers (scales) causes peeling of the scales during rolling, thereby damaging the cold rolling roll and causing bite defects in the surface of the strip. In order to remove such harmful scales, a pickling step is performed after the hot rolling step in a production line for stainless steel. However, since the scales of the hot-rolled stainless steel strip are dense and have poor descalability by pickling, the pickling rate must be decreased, thereby decreasing productivity.
In order to increase the pickling rate by improving the descalability, shot blasting, in which the scales are cracked by spraying hard fine particles (shot particles) on the surface of the steel strip under high pressure, is frequently performed before pickling. However, unevenness (referred to as "shot blast marks") results on the surface of the shot-blast strip, and thus the surface roughness is increased, thereby deteriorating surface quality. It is therefore difficult to use a pickled hot-rolled stainless steel strip as a substitute for a cold-rolled steel strip.
In order to solve these problems, it has been proposed to mechanically apply several percent bending stress to the strip, or to use a polishing brush to decrease the residual shot blast marks (shot blast marks remaining after pickling). However, such methods cannot completely remove the shot blast marks. In addition, such shot blast marks remain after cold-rolling, and cause an undesired reduction in the surface glossiness of the cold-rolled steel strip.
On the other hand, in order to improve the descalability, a procedure for thinning the scales is desirable, and a method of suppressing scaling in the hot rolling step has been proposed. For example, Japanese Patent Unexamined Publication Nos. 58-53323, 59-97710 and 61-123403 disclose a method comprising providing a box whose interior has a controlled atmosphere of inert gas or reducing gas in the region between the outlet side of a final rolling mill and a coiler, with the hot-rolled steel strip being passed through the box after rolling.
In this method, only the scales produced after the final pass remain after coiling, and the scales produced before the final pass are removed in each pass of hot rolling. Therefore, this is based on the technique of controlling the scale thickness by maintaining the strip passage region between the outlet side of the final rolling mill and the coiler in a non-oxidizing atmosphere in order to prevent scaling in this region. However, this method requires gas sealing over the entire wide region between the final rolling mill and the coiler. This technique therefore is quite costly, as it requires installation of a gas sealing apparatus, and supply of a large amount of gas.
Another known method involves removing the scales produced between heating of a slab and hot rough rolling, especially the red scales harmful to pickling, so as to decrease the amount of the scales remaining after pickling. For example, Japanese Patent Unexamined Publication No. 6-71330 discloses a method of descaling an austenitic stainless steel sheet by spraying high-pressure water on the surface of the steel sheet at an impact pressure of 20 to 180 g/mm.sup.2 and a flow rate of 0.1 to 0.6 l/(min.mm.sup.2) before hot finish rolling. Although this method can decrease the amount of scale defects caused mainly by Si oxides, it cannot completely eliminate the scale defects. Also, this method does not permit increasing the pickling rate or achieving pickling without shot blasting. When the method disclosed in Japanese Patent Unexamined Publication No. 6-71330 is applied to a ferritic stainless steel sheet, baking occurs due to metal contact between the roll surfaces of the finish rolling rolls and the surface of the steel sheet, thereby causing the problem of surface defects.
Furthermore, Japanese Patent Unexamined Publication No. 8-108210 discloses a method of producing a hot-rolled ferritic stainless steel strip comprising descaling by spraying high-pressure water on the surface of the steel strip with impact energy (kJ/m.sup.2)of -6.00.times.10-6T+8.60 .times.10! or more, wherein T indicates the temperature (.degree. C.) of the steel strip immediately before descaling, between the end of hot finish rolling and coiling.
However, this method requires a high flow rate of water for obtaining high impact energy, and has the drawback of significantly increasing the size of the associated equipment. Also, since high-pressure water is sprayed after the steel strip is thinned, the surface of the steel strip is locally deformed, and thus the shape of the steel strip becomes unstable, thereby causing difficulty during rolling in some cases.
On the other hand, if a hot-rolled stainless steel strip having scales can be used for applications in which little attention is given to the surface properties, then the pickling step can be omitted, and thus significant cost reduction is expected. However, if the hot-rolled stainless steel strip having scales produced by a conventional process is subjected to molding using a mold, such as bending, drawing, or the like, the scales are partially peeled off, thereby causing the problem of deteriorating the life of the mold and polluting the working environment due to the scattered dust.